Put a big (15'') adjustable wrench on it so you can get some leverage, when it comes off, turn it upside down and spray it down with WD40. Work it on and off the filler tube, spray somemore WD, keep working it until it losenes up and works right. Wipe off the filler tube and excess WD from the cap, have a good day.

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If it ain't broke take it apart and find out why.

1983 300SD, 4 speed
1994 C280
1987 300TD wagon
1996 HD Road King

Ride in Peace Eric Peterson, Harley of Macon